Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. LEDs have higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources, and still higher conversion efficiencies have been demonstrated in the laboratory.
Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is often utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, green, and blue emitting LEDs. The ratio of the intensities of the various colors sets the color of the light as perceived by a human observer.
To replace conventional lighting systems, LED-based sources that generate light that appears to be “white” to a human observer are required. A light source that appears to be white and that has a conversion efficiency comparable to that of fluorescent light sources can be constructed from a blue LED that is covered with a layer of phosphor that converts a portion of the blue light to yellow light. Such light sources will be referred to as “phosphor converted” light sources in the following discussion. If the ratio of blue to yellow light is chosen correctly, the resultant light source appears white to a human observer.
Unfortunately, the uniformity of such phosphor converted light sources presents problems, particularly when two white LEDs are used to illuminate displays that are viewed simultaneously by an observer. Not all white light sources appear the same. For example, incandescent lights emit a spectrum that is approximated by a black body heated to a “color temperature”. If the lights are operated such that the color temperature is high, the white light appears more bluish. If the color temperature is low, the light appears to be more reddish and is perceived to be “warmer” than the higher color temperature light.
White LEDs also vary in their effective color temperature depending on the specific phosphor used to convert the blue light and the amount of phosphor that covers the LED. If too little phosphor covers the LED, the light source appears bluish, since a greater quantity of blue light will escape the LED without being converted. Similarly, if the phosphor layer is too thick, the light source will appear yellowish, since too much of the blue light will have been converted.
The amount of phosphor that overlies the LED die and the manner in which that phosphor is illuminated can vary significantly during the manufacturing process from batch to batch as well as between light sources fabricated in the same batch. As a result, individual LEDs can vary significantly in their effective “color temperature”. If two LEDs that differ significantly from one another are used to illuminate displays that are viewed simultaneously by a human observer, the differences in the emitted spectra are often objectionable to the observer.
A number of solutions have been proposed to reduce the magnitude of this problem. The simplest solution is to sort the LEDs into groups that have similar color temperatures. However, such sorting involves additional tests and increases the inventory problems associated with the manufacture of light sources.
Another solution involves combining a white LED with two or more non-phosphor converted LEDs to produce a light source in which the additional LEDs are used to tune the effective color temperature of the source. For example, U.S. patent application Ser. No. 11/086,138 teaches a scheme in which two red LEDs are combined with a white light source to produce a light source with a controllable color temperature. Similarly, co-pending U.S. patent application Ser. No. 11/523,409 teaches a controllable color temperature white light source that utilizes a white LED together with red, blue, and green LEDs in which the red, blue, and green LEDs are used to tune the color temperature.
These solutions, however, lead to a light source having lower light conversion efficiency than that of the phosphor converted white LEDs. Light conversion efficiency is an important factor in light source design. For the purposes of this discussion, the light conversion efficiency of a light source is defined to be the amount of light generated per watt of electricity consumed by the light source. The presently available phosphor converted white light sources have achieved light conversion efficiencies that are better than those of fluorescent lamps that generate white light. These high light conversion efficiencies are the result of improvements in blue LEDs. The light conversion efficiency of other types of LEDs is lower, and hence, using a combination of phosphor converted white LEDs and non-blue LEDs leads to a light source having a lower overall light conversion efficiency.
Yet another solution is taught in U.S. Pat. No. 7,066,623. This solution utilizes an arrangement in which the various white LEDs are generated with somewhat different blue LEDs to produce LEDs that vary in color about the black body curve. A compound light source having plurality of these off-white light sources is then constructed by testing each LED and grouping the LEDs such that the off-white properties of the LEDs effectively cancel when the LEDs are powered at the same current level. At least one LED from each color grouping is incorporated in the light source to assure that the various LEDs lie on both sides of the black body radiation curve. Hence, the resultant LED appears to be pure white with an intensity equal to that of several white LEDs. This solution requires that the LEDs be both tested and carefully matched. The matching process is inefficient and time consuming. In addition, the color temperature of the final white light source cannot be closely controlled without further sorting and grouping.